1. Field of the Invention
The invention is directed to a photosensor for color measurement based on three spectral components, the sensor chip having three partial surfaces of different sensitivity for detecting the three spectral components through a preceding interference filter structure. The partial surfaces provide measurement values which are converted into color values in a suitable color space, particularly for highly accurate color measurement (point measurement) for testing and guaranteeing color constancy in engineered surfaces and consumer articles of any kind.
2. Description of the Related Art
Over the course of general technical/technological development, requirements for simple individual color sensors, color sensor arrays and color cameras have also increased sharply. This is the result of heightened expectations with regard to design (chiefly color constancy) of industrial products and consumer articles on the one hand and of the steadily increasing demands on quality in man-machine communications on the other hand. The latter branch of technology in particular is characterized by steadily increasing demands of modem media in general and e-commerce in particular and must deliver satisfactory results in a wide variety of display systems and printer systems. For this reason, the push for standardized color measurement (true color) in different application-specific requirements for accuracy continues to increase.
In principle, three different methods for color measurement are known:                comparison methods,        spectral methods, and        tristimulus or three-range methods.        
For reasons of expenditure and cost, the three-range method is relied upon principally, although it has substantial limitations due to the fact that it results in wavelength-integral color values which are valid only for the illumination used. For example, metamerism indices can only be determined through a series of measurements with different illuminations (reference illuminant and test illuminant) which must always be physically present.
The underlying principle of a spectrally adapted semiconductor sensor is already known from U.S. Pat. No. 3,996,461 which describes an optical thin-film filter based on a multilayer interference system for a silicon photosensor for limiting the sensitivity or response of the sensor to the spectral response of the human eye. The interference filter comprises an alternating layer system of pure quarter-wave layers for the wavelengths λ=550 nm, 780 nm and 1000 nm. The high-refraction layers are made of titanium oxide and the low-refraction layers are made of silicon oxide. The resulting filter system corresponds to a Y-characteristic of the spectral response curve of the human eye without the possibility of a spectrally selective resolution of the color spectrum in the sense of a correlation of color values or standard spectral colors, since the filter layers are not structured but rather are applied to a plurality of sensor elements at the same time.
U.S. Pat. No. 5,246,803 discloses structured or patterned dichroic filters for solid state electronic image sensors which are carried out by alternating deposition on the sensor surface or on a glass layer. The description refers to the alternating deposition of SiO2 layers and TiO2 layers under vacuum conditions and low temperature for generating color filters. The spectral characteristic of the filters is controlled through the quantity and thickness of the layers and the shaping and deposition of the layers is repeated as often as necessary to generate red filters, green filters and blue filters. According to U.S. Pat. No. 5,246,803, for example, in an alternating layer filter stack for the blue filter, a pass region (“on-band region” with approximately 80% transmission) of 400–500 nm is generated, while the reflection region (“off-band region” with less than 5% transmission) is between 500 and 700 nm.
This solution is disadvantageous in that it involves pure bandpass or edge filters, so that point measurements with narrowband color stimuli in the off-band region of the color filters regularly lead to the measurement of falsified color values or require a special reference light calibration.